1. Field of the Invention
This invention relates to apparatus for producing duplex copies. More particularly, this invention relates to apparatus for transferring unfixed transferable images to both sides of a copy sheet before fixing of either image to the copy sheet.
2. Description of the Prior Art
Several techniques are known in the prior art for forming duplex images on a final copy medium such as a web or sheet. One such technique requires the use of two photoconductors upon which first and second transferable images are formed respectively. The copy medium is generally passed between the photoconductors and the first and second images are transferred to opposite sides of the copy medium. Another technique similar to the above but involving the use of only one photoconductor, utilizes an intermediate image storage member such as a drum, to receive the first image formed on the photoconductor before transfer to a final copy medium. These techniques suffer the disadvantages of increased cost, machine complexity and size and decreased reliability necessitated by the use either of two photoconductive and optical systems or of additional components before transfer to a final copy medium. Additionally there is the probability of degradation in image quality when an intermediate storage member is used. Furthermore, in some instances the developed images are tackified by use of solvent vapors which are potentially flammable and which require the use of a consumable fluid which must be replaced periodically.
A further duplexing technique utilized in certain commercial electrophotographic machines includes a single photoconductor wherein first fixed images developed sequentially on the first sides of a plurality of copy sheets by an electrophotographic process are collected in an intermediate tray. The copy sheets are then sequentially transported back through the electrophotographic process to develop second fixed images on the second sides of the copy sheets, thus producing duplex copies. The latter "two-pass" process has several disadvantages. Because the first sides of all the copy sheets are developed before development of the second sides of the copy sheets, a duplex copy is not available for inspection until all of the first sides of the copy sheets and one set of the second sides thereof have been developed. In addition, the relatively long paper paths required in passing a copy sheet through the entire electrophotographic process twice greatly increases the possibility of paper jams and other potential copy handling complications. Moreover, environmental conditions of image formation and the physical parameters of the copy sheet may change resulting in images of varying quality on opposite sides of a single sheet and misregistration between images on opposite sides of a copy sheet may also result.
Still another duplex copying technique which may be considered especially relevant to the present invention involves fixing images to both sides of a copy sheet during a single pass through the disclosed electrophotographic processes. U.S. Pat. Nos. 3,506,347; 3,672,765; 3,869,202; and 3,947,270 disclose various embodiments of this technique. In the first patent just listed a first tackified image is formed on a transfer drum, the image is transferred to the first side of a copy sheet, and the sheet is inverted while the first tackified image dries and becomes fixed on the copy sheet. Thereafter, a second tackified copy image is formed on the transfer drum and the copy sheet is fed back into contact with the drum to transfer the second side of the copy sheet which is then transported to an output tray. In the latter three patents electrophotographic apparatus is disclosed for making copies wherein two images of an original are formed sequentially on a photoconductor, the images are developed and the first developed image is transferred to the first side of a copy sheet. The copy sheet is passed through a fuser to fuse or fix the first transferred image, is turned over and the opposite side of the copy sheet is brought into contact with the second developed image on the photoconductor. The second image is then transferred to the second side of the copy sheet, the copy sheet separated from the photoconductor and the second image fused by means of a second fuser. The disclosed techniques have several disadvantages. Since the first image is fixed before transfer of the second image, either two fusers must be used with attendant increase in cost, power and environmental heat or solvent fixing is used resulting in safety hazards and the inconvenience of handling consumable liquids.
Still another technique is disclosed in Research Disclosure, April, 1977, Pages 80-81, Item No. 15671, (published by Industrial Opportunities Ltd., Homewell, Havant Hampshire, United Kingdom) discloses a vacuum belt for inverting a copy sheet having a simplex unfixed image, the inverter being positioned between first and second transfer stations. The belt is driven in both clockwise and counterclockwise directions at film speed in order to transport the copy sheet in its turnover path at film speed. This technique may result in long interframe distances (i.e. the distance between successive image frames on the photoconductive member) due to space and time limitations to effect copy sheet turnover. Long interframe distances have disadvantages with respect to the efficiency and copy output for a given velocity of the photoconductive member.